Add raw protobuf kafka support

[devinrsmith]

The existing kafka protobuf feature relies on the schema registry and the kafka protobuf serdes protocol.

Some use cases dictate the parsing of raw protobuf from a topic - ie, a topic that does not have the magic byte, or the 4 byte schema id. It should be possible to add efficient DH support for raw parsing based on a Descriptor:

protobuf_spec(
    ...
    schema_message_name = "com.example.MyMessage",
    type = "local" # or "serdes" (the default), or "registry"
)

serdes would be the default, and would mean the schema exists on the registry and the topic is the "serdes" protocol
local would mean the schema exists on the classpath and the topic is a "raw" protocol
registry would mean the schema exists on the registry and the topic is a "raw" protocol

[devinrsmith]

It might also be reasonable to add "local" support, or point-in-time "registry" support, even if the protocol is really serdes. This might allow DH to be more efficient and skip the confluent io.confluent.kafka.serializers.protobuf.KafkaProtobufDeserializer impl.

[devinrsmith]

Fixed by #4585